The present invention relates to a control device for controlling the laying of pipe at the bottom of a body of deep water, including a combined pipe displacement and pipe tension control, of the type in which a holding force is exerted on the pipeline by a tensioner and the pipeline is let out in sections of predetermined length, the pipe laying barge being moved in a stepwise manner along the laying path in the laying direction.
The pipe is formed by welding successive pipe sections on the pipe laying barge to the pipe which has already been formed and which must be installed at the bottom of the ocean, or other water body, on a given line, or path. The pipeline is lowered over a rigid stern ramp. A tensioner keeps the pipeline to be lowered under a constant tension so that bending stresses exerted on, and curvatures of, the length of pipe suspended between the barge and the ocean bottom will not exceed predetermined limits. Such a tensioner can be constituted by a clamping device which produces a frictional sliding or rolling engagement between the pipe and pipe layer to permit advancement of the pipe laying barge with constant holding force on the pipe. The principles underlying such an arrangement are described in "Wissenschaftlicher Bericht" [Scientific Report] by AEG-Telefunken 49 (1976), pages 50-59, and Hansa Schiffahrt-Schiffbau-Hafen, Volume 112, 1975, No. 12, pages 983-986.
The driving and holding forces applied to the pipe laying barge are generated, for example, by mooring winches acting via anchors and associated cables, the anchor cables being extendable to about 3500 to 4000 meters.
The pipe laying barge is equipped with a welding line in which the pipe sections are welded to the pipe being laid in several processing stages. When the welding process is completed, the pipe laying barge is moved forward by the length of one pipe section. The clamping conditions at the stern ramp and at the tensioner must be such as to assure that undue deformations of the pipeline are definitely avoided. After having payed out the pipe section by moving forward the pipe laying barge by means of the bow winches, the pipe laying barge is braked to zero by means of the stern winches.
This sequence is repeated after each succeeding pipe section has been welded to the pipe.
In this pipe laying procedure the holding force, or tension, on the pipe is kept constant within narrow preset limits and there is as little movement as possible of the pipe relative to the pipe laying barge as long as the pipe tension remains within these limits. The control characteristic of the tensioner is designed in this case so that the value of the pipe tension, or holding force, is composed of a constant component and a component proportional to the relative speed between the pipe and pipe laying barge.
Considering the procedure in greater detail, during welding of a new pipe section to the previously formed pipeline, the speed of the barge in the forward direction is zero. The barge is held stationary by means of the anchor winches. At the same time the pipeline is kept stationary relative to the barge by causing the tensioner to apply to the pipeline a holding force which remains within preset limits.
If the barge should be subjected to substantial disturbance forces, such as heavy seas or strong winds, the barge will experience displacements which cannot be compensated for quickly enough by action of the anchor cable winches. Therefore, in order to prevent damage to the pipeline under such conditions, pipe must be payed out if the holding force goes above the preset upper limit, or hauled in if the holding force goes below the preset lower limit. Only if the barge movements are small can the pipeline remain stationary relative to the barge.
To satisfy these requirements, the tensioner control device is designed to present a dead zone in its control characteristic. This dead zone is delimited by the above-mentioned preset upper and lower holding force limits in that as long as the holding force remains within these limits, the pipeline is held stationary relative to the barge, while if the holding force goes below or above the dead zone range the pipeline is hauled in or payed out.
As a result of keeping the pipe tension constant within these close, predetermined limits and slowly advancing the pipe laying barge, the speed of the pipe movement relative to the pipe laying barge is also slow and the time required to lay the pipeline is considerable.